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Preparation

Bring the milk, water, and 1 tablespoon salt to a boil in a large pot--keep an eye on the pot, because milk has a tendency to bubble up furiously. Drop in the celery root, potato, and onion, adjust the heat so that the liquid simmers steadily, and cook for about 30 minutes, or until the vegetables are tender and can be pierced easily with the point of a knife. Drain, discard the liquid, and shake the colander well to remove as much liquid as possible.
In a food processor, in batches if necessary, puree the vegetables until perfectly smooth. Add the butter and whir until completely incorporated. Taste and add salt and white pepper as needed.
Transfer the puree to a warm serving bowl and, if you'd like, top with a generous shower of chopped chives or a drizzle of pistachio oil or butter that's been cooked over low heat until its color is honey brown. Serve immediately.